CMOS power oscillator with frequency modulation

ABSTRACT

CMOS power oscillator and a method of frequency modulating a CMOS power oscillator. The oscillator comprises a transformer-based feedback CMOS power oscillator circuit formed on a chip-substrate, the oscillator circuit including a transformer coupled to a transistor; means for modulating the capacitance of the transformer to the chip-substrate for frequency modulating an output of the power oscillator.

FIELD OF INVENTION

The present invention relates broadly to a complementary metal oxidesemiconductor (CMOS) power oscillator with frequency modulation, and toa method of frequency modulating a CMOS power oscillator.

BACKGROUND

The rapidly growing market of personal communication systems, radiomedical implanted systems, and wireless hearing aids provides anincreasing demand for more integrated and more efficient radio frequency(RF) integrated circuits (IC's). These IC's are required to operate withsupply voltages under 2V and sometimes down to 1V with minimum currentconsumption at frequencies up to several GHz. Such applicationstypically contain a combination of several modules including a poweramplifier, an oscillator, for example a voltage controlled oscillator(VCO), and modulator.

For example Class E power amplifier circuits are very suitable for highefficiency power amplification applications in the radio-frequency andmicrowave ranges. However, due to the inherent asymmetrical drivingarrangement, existing Class E amplifier circuits suffer significantharmonic contents in the output voltage and current, and usually requiresubstantial design efforts in achieving the desired load matchingnetworks for applications requiring very low harmonic contents.

The basic Class E circuit is typically implemented using discretecomponents including a transistor, which is connected with an RFC to thesupply voltage and to the load network. The load network is made up of acapacitor shunting the transistor and a series tuned inductor capacitorresonant circuit. The transistor is driven hard enough to act like aswitch. The principle of Class E power amplifiers is to avoid by designthe simultaneous existence of high voltage and high current in theswitch, even in the case of a long switching time. That would imply 100%efficient conversion of dc to RF energy.

Frequency modulation is typically implemented via a varactor and isbased on an LC-tank circuit. However, this requires additional discretecomponents to match the load network resulting in lower powerefficiency. Typical solutions include using two identical resonantcircuits, which encounters the same problem of matching inductors andcapacitors, as well as using symmetrically driven push-pull Class Eamplifier for high power applications.

A need therefore exist for providing an alternative oscillator designwith frequency modulation capability, which seeks to address one or moreof the above mentioned problems.

SUMMARY

In accordance with a first aspect of the present invention there isprovided a CMOS power oscillator comprising a transformer-based feedbackCMOS power oscillator circuit formed on a chip-substrate, the oscillatorcircuit including a transformer coupled to a transistor; means formodulating the capacitance of the transformer to the chip-substrate forfrequency modulating an output of the power oscillator.

The means for modulating may comprise a patterned ground shield (PGS)layer formed in the chip-substrate and coupled to an input circuit forreceiving a modulating signal.

The power oscillator may further comprise a conducting layer formed inthe chip-substrate for shielding the PGS layer and the transformer.

The input circuit may comprise a MOS FET.

The modulating signal may alternately set the PGS to floating and togrounding for modulating the capacitance of the transformer to thechip-substrate for frequency modulating an output of the poweroscillator.

The means for modulating may comprise a deep N-well formed in thechip-substrate and coupled to an input circuit for receiving of amodulating signal.

The deep N-well may comprise a p-n junction.

The modulating signal may alternate the p-n junction capacitance andresistance for modulating the capacitance of the transformer to thechip-substrate for frequency modulating an output of the poweroscillator.

The oscillator circuit may further include a variable capacitancecoupled between an output terminal of the power oscillator and groundfor varying an output carrier frequency of the power oscillator.

The variable capacitor may comprise a varactor for implementing avoltage controlled oscillator (VCO) with frequency modulationcapabilities.

The transformer may provide a feedback path between the drain and thegate of the transistor.

A first port of the transformer may be connected for RF grounding anddrain bias feeding, and a second port of the transformer is connectedfor RF grounding an gate bias feeding.

A third port of the transformer may be connected to the drain of thetransistor, and a fourth port of the transformer is connected to thegate of the transistor.

Parameters of the transistor and parameters of the transformer may bechosen to pre-set the output carrier frequency of the power oscillator.

In accordance with a second aspect of the present invention there isprovided a method of frequency modulating a CMOS power oscillator, themethod comprising providing a transformer-based feedback CMOS poweroscillator circuit formed on a chip-substrate, the oscillator circuitincluding a transformer coupled to a transistor; and modulating thecapacitance of the transformer to the chip-substrate for frequencymodulating an output of the power oscillator.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be better understood and readilyapparent to one of ordinary skill in the art from the following writtendescription, by way of example only, and in conjunction with thedrawings, in which:

FIGS. 1 (a) and (b) show a circuit schematic and a die microphotographrespectively of a CMOS process technology oscillator with FM modulation.

FIG. 2 shows the on-chip transformer equivalent circuit for theoscillator of FIG. 1.

FIG. 3 is a graph showing the simulated waveforms of the output voltageand current for the oscillator of FIG. 1.

FIG. 4 shows a measured output spectrum of the oscillator of FIG. 1.

FIG. 5 shows the carrier frequency and DC-to-RF conversion efficiency asa function of gate voltage for the oscillator of FIG. 1.

FIG. 6 shows a schematic cross-sectional view of the transformer of FIG.1.

FIG. 7 shows a schematic circuit diagram illustrating modulation of thecapacitance of the transformer to the chip-substrate of the oscillatorof FIG. 1.

FIG. 8 shows a FM signal spectrum for the oscillator of FIG. 1.

FIGS. 9 shows a circuit schematic of a CMOS process technology VCO withFM modulation.

FIG. 10 shows the simulated frequency and output power as a function ofcontrolled voltage of the VCO of FIG. 9.

FIG. 11 shows a schematic cross-sectional view of the transformer ofFIG. 9.

FIG. 12 shows a FM signal spectrum for the VCO of FIG. 9.

FIG. 13 shows a schematic cross-sectional view of a transformer with PGSmetals for FM modulation.

FIG. 14 shows a schematic cross-sectional view of a transformer withdeep N-well for FM modulation.

FIG. 15 shows a flowchart illustrating a method of frequency modulatinga CMOS power oscillator.

DETAILED DESCRIPTION

FIGS. 1 (a) and (b) show a circuit schematic and a die microphotographrespectively of an on-chip power oscillator structure 100. The structure100 is fabricated using a conventional 0.18 μm CMOS process technology,with six metal TiW/Al-1% Si/TiW interconnects on a lossy siliconsubstrate 102 of 10 Ωcm. A three and a half turn circular spiraltransformer 104 with metal trace width of 10 μm, a spacing of 2 μm andan inner diameter of 100 μm (total size: about 270×270 μm²) is formed onthe substrate 102 (FIG. 1 (b)). The transformer metal traces 106 areformed by a top metal layer of 2 μm thickness, and two embedded metallayers on the substrate 102 are stacked together with a dense resistivevia array 108 to form an underpass.

The input and output ports 112, 114 of the transformer 104 are connectedto respective ground-signal-ground (GSG) pads 116. A ground guard-ringstructure 118 is laid out for better grounding. The equivalent circuit200 for the on-chip transformer 104 is presented in FIG. 2. The circuit200 consists of three parts: I) self-inductances, self-resistances (L1,L2, R1, R2); II) coupling capacitances (C12, C13, C23), and III)substrate effect parasitics, including oxide capacitances (Cs1, Cs2,Cs3), substrate capacitances (Cs11, Cs22, Cs33) and substrateresistances (Rs1, Rs2, Rs3). The mutual inductance between the metaltraces is described by parameter K. Accurate parameters of thetransformer circuit 200 model can be easily extracted from measuredS-parameters. The feedback topology is chosen for a power oscillatordesign.

Returning to FIG. 1( b), the on-chip transformer 104 is used as a RFsignal feedback and bias supply paths between the drain 119 and gate 120of a power transistor 122 to reduce substrate coupling and resistanceloss to achieve a high efficiency. Ports 117, 121 of the primary andsecondary sides of the on-chip transformer 104 are connected to thedrain 119 and the gate 120 of the CMOS power transistor 122,respectively. The ports 114, 112 are connected to capacitors 128, 130respectively for RF grounding, as well as for drain and gate biasfeeding, respectively. The output port of the power oscillator 100 isfrom the RF terminal 132 connected to the drain 119 via capacitor 134.

For considerations of the circuit design, the size of the transistor 122and the number of turns of the transformer 104 determines theoscillating carrier frequency, as the transformer provides a feedbackpath that forms a resonant loop for the desired oscillating carrierfrequency. The size of the transistor 122 also determines the RF outputpower level (calculation based on transistor P_(outmax)<about 0.1 W/mmand efficiency), with a larger size transistor 122 providing more powergain to the oscillator 100, while the operating carrier frequencydecreases due to a higher C_(gs). Therefore, the output power andoperating frequency are a trade-off between dimensions of thetransformer 104 and the size of the transistor 122.

An NMOS transistor 122 with gate length of 0.18 μm and total width of550 μm is used for the power oscillator 100. The transistor RF model iscreated using a Bsim3 model for simulation together with extractedsubstrate and gate network parameters. Simulation was carried out usingextracted RF models of the transistor 122, the transformer 104 and thecapacitors 128, 130, 134. The waveforms of the output voltage (curve300) and current (curve 302) are shown in FIG. 3. The voltage waveform(curve 300) shows that the circuit operates in Class-E mode. The carrierfrequency is at about 2.45 GHz with an output power of 15.5 dBm and aphase noise of about −122 dBc/Hz at 100 kHz offset at V_(ds)=1.8 V andI_(ds)=29.8 mA.

The fabricated oscillator 100 with a die size of 0.6×0.7 mm with the GSGtest pad 116, was also measured using a HP 8563E spectrum analyzer withphase noise measurement option and battery power supply. The oscillator100 was placed in a small shielded chamber during the measurements. Themeasured results shown in FIG. 4 demonstrate that the output power isabout 15.3 dBm with a phase noise of about −113 dBc/Hz at 100 kHz offsetfrom a carrier frequency of about 2.446 GHz at V_(ds)=1.8 V andI_(ds)=28.7 mA.

The carrier frequency (curve 500) and DC-to-RF conversion efficiency(curve 502) as a function of gate voltage were also measured and areshown in FIG. 5. The results show that the carrier frequency drifts downslightly while the gate voltage increases from 0.47 to 0.89 V, and thepeak efficiency of the DC-to-RF conversion of about 66% occurs atV_(gs)=0.71 V. This is believed to be due to the increase intransconductance, g_(m), resulting in an increase in feedback powerlevel while the gate voltage increases. The nonlinear part in the outputspectrum, especially the third-harmonic signal, will reduce the carrieroutput power at higher gate voltages, as the gate voltage increases, andthe increase in gate capacitance induces a decrease in carrierfrequency. Further increase in gate voltage results in multi-oscillatingfrequencies.

Returning now to FIG. 1( b), the oscillator 100 can be used as a Class Epower amplified type circuit with the oscillator in switching mode, andexhibits low phase noise, high efficiency and high power. Thetransformer 104 is used to generate a feedback path to meet theoscillation loop requirement: loop-phase equal to 360° and amplitude isgreater than 1, while the transistor 122 provides the loop power gainand part of the loop phase shift. This results in a Class E poweramplified type circuit with very low phase noise.

The oscillator 100 can be modulated by feeding a modulating signal to aPatterned Ground Shield (PGS) layer 600 of the substrate 102 (visiblethrough transparent oxide layers of the substrate 102) which affects thetransformer 104 on the top layer, thus forming an oscillator withmodulation. PGS layers are typically used for isolating a circuit on topof a substrate from the rest of the substrate and around the circuit.FIG. 6 shows a schematic cross-sectional view of the transformer 104,illustrating the location of the PGS layer 600 underneath thetransformer 104. The modulation signal 602 is provided to the PGS layer600 via a transistor 604. The modulation signal 602 is utilized tomodulate the gate of the transistor 604 and to modulate the channelresistance. While the channel resistance is very high, the PGS layer 600behaves as a floating metal layer, and while the channel resistance isvery low, the PGS layer 600 behaves as if it is connected to ground. Dueto the proximity of the PGS layer 600 to the transformer 104, themodulation signal 602 will modulate the distance between the transformer104 and ground 606 to change the capacitance between the transformer 104and the substrate 102, which in turn modulates the effective inductanceof the transformer 104.

The PGS layer 600 can be set to floating or grounding to modulate thecapacitance of the transformer 104 to the substrate 102. The effectiveinductance of the transformer 104 is modulated by the modulating signal602 and the carrier frequency of the oscillator 100 is thus modulated bythe modulating signal. FIG. 7 is a schematic circuit diagramillustrating the influence of the modulated capacitance of thetransformer 104 to the substrate 102, indicated as arrows 700 to 704 inFIG. 7. FIG. 8 shows an FM signal spectrum 800 for a modulating signalhaving a pulse frequency of about 30 kHz and width of about 900 ns, anda modulated voltage of about 0.7V while the drain voltage is about 1.5Vand the gate voltage is about 0.7V.

FIG. 9 shows a circuit schematic of a CMOS process technology VCOstructure 900 with FM modulation, which is a modification of theoscillator structure 100 of FIG. 1( a). The modification consists ofconnecting a variable capacitor in the form of a MOS varactor 902between the RF output 904 and the RF ground 906. FIG. 10 shows thesimulated frequency (curve 1000) and output power (curve 1002)respectively as a function of the controlled voltage applied to the MOSvaractor (compare 902 in FIG. 9). FIG. 10 demonstrates that the circuitcan function as a VCO. Further simulations showed that if the variablecapacitor (compare 902 in FIG. 9) changes from 0.1 pf to 4 pf, theoscillating frequency changes from about 2.45 GHz to about 1.39 GHz,with the drain voltage at about 1.5V and the gate voltage at about 0.7V.

The VCO can again be modulated by feeding a modulating signal to aPatterned Ground Shield (PGS) layer of the substrate which affects thetransformer, thus forming a VCO with modulation. FIG. 11 shows aschematic cross-sectional view of the transformer 1102, illustrating thelocation of the PGS layer 1100 underneath the transformer 1102. Themodulation signal 1104 is provided to the PGS layer 1100 via atransistor structure 1106. The PGS layer 1100 can be set to floating orgrounding to modulate the capacitance of the transformer 1102 to thesubstrate 1108. The effective inductance of the transformer 1102 ismodulated by the modulating signal 1104 and the carrier frequency of theVCO 900 is thus modulated by the modulating signal. FIG. 12 shows afrequency modulated (FM) signal spectrum 1200 with a modulation signalfrequency of about 200 Hz and amplitude about 1V with an offset of about0.5V, while the drain voltage is about 1.5V and the gate voltage isabout 0.7V.

In another arrangement illustrated in FIG. 13, an additional metal layer1300 may be provided in conjunction with a PGS layer 1302 for providingelectrical isolation, since the PGS layer 1300 is being used for themodulating signal 1306. This arrangement is suitable for applicationswhere isolation of the oscillator or VCO circuit, represented bytransformer 1308, is important. As shown in FIG. 14, in anotherarrangement, an oscillator or VCO circuit, represented by transformer1400, can be modulated by utilizing a deep N-well (DNVV) 1402 within asubstrate 1404 to control the oscillator or VCO. In this embodiment, amodulation signal 1406 is directly provided to the deep N-well 1402,which operates as a P-N junction. The modulation signal 1406 modulatesthe P-N junction capacitance and resistance, which in turn modulates thesubstrate 1404 capacitance and resistance and thus the transformer 1400to substrate 1404 capacitance and resistance. The P-N junction is formedbetween N-well 1402 and the P-type substrate 1404, with the P-typesubstrate providing grounding.

FIG. 15 shows a flowchart 1500 illustrating a method of frequencymodulating a CMOS power oscillator. At step 1502, a transformer-basedfeedback CMOS power oscillator circuit formed on a chip-substrate isprovided, the oscillator circuit including a transformer coupled to atransistor. At step 1504, the capacitance of the transformer to thechip-substrate is modulated for frequency modulating an output of thepower oscillator.

The combination of a CMOS oscillator or VCO with a method of modulatingthe oscillator or VCO in the described arrangements can result in adevice that is suitable for small applications due to fewer componentsbeing used compared to existing devices, with high power, highefficiency and low phase noise. The overall size is reduced due toutilising the CMOS-based combination of a power amplifier, oscillatorand modulator. The device is cost effective and can be used in e.g.simple transceiver applications as well as remote controls and Bluetoothapplications.

It will be appreciated by a person skilled in the art that numerousvariations and/or modifications may be made to the present invention asshown in the specific embodiments without departing from the spirit orscope of the invention as broadly described. The present embodimentsare, therefore, to be considered in all respects to be illustrative andnot restrictive.

1. A CMOS power oscillator comprising: a transformer-based feedback CMOSpower oscillator circuit formed on a chip-substrate, the oscillatorcircuit including a transformer coupled to a transistor; means formodulating the capacitance of the transformer to the chip-substrate forfrequency modulating an output of the power oscillator.
 2. The poweroscillator as claimed in claim 1 wherein the means for modulatingcomprises a patterned ground shield (PGS) layer formed in thechip-substrate and coupled to an input circuit for receiving amodulating signal.
 3. The power oscillator as claimed in claim 2,further comprising a conducting layer formed in the chip-substrate forshielding the PGS layer and the transformer.
 4. The power oscillator asclaimed in claim 3, wherein the input circuit comprises a MOS FET. 5.The power oscillator as claimed in claim 3, wherein the modulatingsignal alternately sets the PGS to floating and to grounding formodulating the capacitance of the transformer to the chip-substrate forfrequency modulating an output of the power oscillator.
 6. The poweroscillator as claimed in claim 2, wherein the input circuit comprises aMOS FET.
 7. The power oscillator as claimed in claim 6, wherein themodulating signal alternately sets the PGS to floating and to groundingfor modulating the capacitance of the transformer to the chip-substratefor frequency modulating an output of the power oscillator.
 8. The poweroscillator as claimed in claim 2, wherein the modulating signalalternately sets the PGS to floating and to grounding for modulating thecapacitance of the transformer to the chip-substrate for frequencymodulating an output of the power oscillator.
 9. The power oscillator asclaimed in claim 1 wherein the means for modulating comprises a deepN-well formed in the chip-substrate and coupled to an input circuit forreceiving of a modulating signal.
 10. The power oscillator as claimed inclaim 9, wherein the deep N-well comprises a p-n junction.
 11. The poweroscillator as claimed in claim 10, wherein the modulating signalalternates the p-n junction capacitance and resistance for modulatingthe capacitance of the transformer to the chip-substrate for frequencymodulating an output of the power oscillator.
 12. The power oscillatoras claimed in claim 1, wherein the oscillator circuit further includes avariable capacitance coupled between an output terminal of the poweroscillator and ground for varying an output carrier frequency of thepower oscillator.
 13. The power oscillator as claimed in claim 12,wherein the variable capacitor comprises a varactor for implementing avoltage controlled oscillator (VCO) with frequency modulationcapabilities.
 14. The power oscillator as claimed in claim 1, whereinthe transformer provides a feedback path between the drain and the gateof the transistor.
 15. The power oscillator as claimed in claim 1,wherein a first port of the transformer is connected for RF groundingand drain bias feeding, and a second port of the transformer isconnected for RF grounding and gate bias feeding.
 16. The poweroscillator as claimed in claim 1, wherein a third port of thetransformer is connected to the drain of the transistor, and a fourthport of the transformer is connected to the gate of the transistor. 17.The power oscillator as claimed in claim 1, wherein parameters of thetransistor and parameters of the transformer are chosen to pre-set theoutput carrier frequency of the power oscillator.